Method and apparatus for monitoring a wireless link in a wireless communication system

ABSTRACT

The present invention relates to a wireless communication system, and more particularly to a method and apparatus for monitoring a wireless link. A method for monitoring a wireless link by a terminal in a wireless communication system according to one embodiment of the present invention comprises the steps of: receiving a precoded control channel; and estimating the quality of the wireless link for the received precoded control channel, wherein the quality of the wireless link can be estimated on the basis of an assumption by the terminal regarding the precoding applied to the precoded control channel.

This application is a reissue of U.S. Pat. No. 9,344,909, which issuedMay 17, 2016 from U.S. patent application Ser. No. 14/131,890, filedJan. 9, 2014, which is a 35 USC §371 National Stage entry ofInternational Application No. PCT/KR2012/005865, filed on Jul. 23, 2012,and claims priority to U.S. Provisional Application Nos. 61/511,505filed Jul. 25, 2011, 61/560,796 filed Nov. 16, 2011 and 61/602,075 filedFeb. 22, 2012, all of which are hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

Embodiments of the present invention relate to a wireless communicationsystem, and more particularly to a method and apparatus for monitoring aradio link.

BACKGROUND ART

The quality of a radio link between a base station (BS) (or eNB) and auser equipment (UE) may be degraded due to various factors. When the UEfails to receive a control signal from the BS (for example, if aphysical downlink control channel (PDCCH) is correctly decoded), thismay be defined as a Radio Link Failure (RLF). To handle the RLF, the UEfirst detects a problem at a physical layer and attempts to solve thephysical layer problem. If the UE fails to recover from the physicallayer problem, the UE may transmit a connection re-establishment requestto the BS, after determining that an RLF has been detected.

DISCLOSURE Technical Problem

In order to maintain/recover connection between the BS and the UE, thereis a need to correctly perform radio link monitoring (RLM). If the RLMresult does not correctly estimate a radio link quality, the UE declaresRLF even in a good radio link quality so that an unnecessary operationoccurs. Alternatively, the UE does not declare RLF even in a poor radiolink quality such that errors of data transmission/reception cannot besolved.

Various techniques for improving control channel (e.g., PDCCH)performance have been proposed in the evolved radio communicationsystem. The probability of PDCCH decoding error in the legacy RLMoperation has been defined in consideration of the legacy PDCCH Tx/Rxtechniques, such that the legacy RLM result may incorrectly reflect theactual link quality under a new PDCCH Tx/Rx technique.

An object of the present invention is to provide a method for correctlyand efficiently performing radio link monitoring (RLM) in considerationof a new control channel Tx/Rx method.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned herein willbe apparent from the following description to one of ordinary skill inthe art to which the present invention pertains.

Technical Solution

The object of the present invention can be achieved by providing amethod for performing radio link monitoring (RLM) by a user equipment(UE) in a wireless communication system, the method including: receivinga precoded control channel; and estimating a radio link quality of thereceived precoded control channel, wherein the radio link quality isestimated on the basis of UE assumption associated with precodingapplied to the precoded control channel.

In another aspect of the present invention, a user equipment (UE) forperforming radio link monitoring (RLM) in a wireless communicationsystem includes: a reception (Rx) module configured to receive adownlink signal from a base station (BS); a transmission (Tx) moduleconfigured to transmit an uplink signal to the BS; and a processorconfigured to control the UE including the reception (Rx) module and thetransmission (Tx) module, wherein the processor receives a precodedcontrol channel through the reception (Rx) module, estimates a radiolink quality of the received precoded control channel, the radio linkquality being estimated on the basis of UE assumption associated withprecoding applied to the precoded control channel.

The following description may be commonly applied to the embodiments ofthe present invention.

The UE assumption may indicate that a precoding matrix optimal for adownlink (DL) channel estimated by the UE is applied to the precodedcontrol channel.

The downlink (DL) channel may be estimated from a Channel StateInformation-Reference Signal (CSI-RS) or a cell-specific referencesignal (RS).

The optimal precoding matrix may correspond to a Precoding MatrixIndicator (PMI) reported from the UE to a base station (BS).

The UE assumption may indicate that a precoding matrix randomly selectedfrom a predetermined codebook is applied to the precoded controlchannel.

A transmission (Tx) rank defined in the predetermined codebook may beequal to or less than a transmission (Tx) rank of the precoded controlchannel.

The UE assumption may indicate that one precoding matrix is applied tothe precoded control channel.

The method may further include: demodulating the precoded controlchannel on the basis of a channel estimated using a UE-specificReference Signal (RS), wherein the same precoding as precoding appliedto the UE-specific RS is applied to the precoded control channel.

Information regarding the precoding applied to the precoded controlchannel may not be applied to the user equipment (UE).

The radio link quality estimation may be based on a decoding errorprobability of the received precoded control channel.

The radio link quality may be estimated on the assumption that at leastone of the number of slots used for transmission of the precoded controlchannel, the number of orthogonal frequency division multiplexing (OFDM)symbols, and the number of resource elements (REs) is constant.

The method may further include: receiving a non-precoded controlchannel; and estimating a radio link quality of the non-precoded controlchannel, wherein a status of the radio link quality is determined bycomparing at least one of a radio link quality of the precoded controlchannel and a radio link quality of the non-precoded control channelwith a predetermined threshold value.

The method may further include: receiving a non-precoded controlchannel; and estimating a radio link quality of the non-precoded controlchannel, wherein a status of the radio link quality is determined bycomparing at least one of a radio link quality of the precoded controlchannel and a radio link quality of the non-precoded control channelwith a predetermined threshold value.

The precoded control channel may be an Enhanced-Physical DownlinkControl Channel (E-PDCCH).

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

As is apparent from the above description, the embodiments of thepresent invention can provide a method for correctly and efficientlyperforming RLM in consideration of a new control channel Tx/Rx method.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved through the present invention are not limited towhat has been particularly described hereinabove and other advantages ofthe present invention will be more clearly understood from the followingdetailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows a radio frame structure.

FIG. 2 exemplarily shows a resource grid of a downlink slot.

FIG. 3 exemplarily shows a downlink subframe structure.

FIG. 4 exemplarily shows an uplink subframe structure.

FIG. 5 is a diagram illustrating a wireless communication systemincluding multiple antennas.

FIG. 6 is a diagram showing exemplary CRS and DRS patterns.

FIG. 7 is a diagram showing exemplary CSI-RS patterns.

FIG. 8 is a diagram showing exemplary DMRS patterns.

FIG. 9 is a diagram showing exemplary E-PDCCH structures.

FIG. 10 is a flowchart illustrating an RLM method according to oneembodiment of the present invention.

FIG. 11 is a block diagram illustrating a transceiver apparatusapplicable to embodiments of the present invention.

FIG. 12 is a flowchart illustrating features of the present invention.

BEST MODE

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered to optional factors on thecondition that there is no additional remark. If required, theindividual constituent components or characteristics may not be combinedwith other components or characteristics. Also, some constituentcomponents and/or characteristics may be combined to implement theembodiments of the present invention. The order of operations to bedisclosed in the embodiments of the present invention may be changed.Some components or characteristics of any embodiment may also beincluded in other embodiments, or may be replaced with those of theother embodiments as necessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “Base Station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access pointas necessary. The term “relay” may be replaced with a Relay Node (RN) ora Relay Station (RS). The term “terminal” may also be replaced with aUser Equipment (UE), a Mobile Station (MS), a Mobile Subscriber Station(MSS) or a Subscriber Station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Project Partnership (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, the steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied through wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as GSM (Global System for Mobile communications)/GPRS (GeneralPacket Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution).OFDMA may be embodied through wireless (or radio) technology such asInstitute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). UTRA is apart of UMTS (Universal Mobile Telecommunications System). 3GPP (3rdGeneration Partnership Project) LTE (long term evolution) is a part ofE-UMTS (Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA indownlink and employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is anevolved version of 3GPP LTE. WiMAX can be explained by an IEEE 802.16e(WirelessMAN-OFDMA Reference System) and an advanced IEEE 802.16m(WirelessMAN-OFDMA Advanced System). For clarity, the followingdescription focuses on 3GPP LTE and 3GPP LTE-A systems. However,technical features of the present invention are not limited thereto.

A radio frame structure will now be described with reference to FIG. 1 .

In a cellular OFDM wireless packet communication system, anuplink/downlink data packet is transmitted on a subframe basis and onesubframe is defined as a predetermined time interval including aplurality of OFDM symbols. 3GPP LTE standard supports a type-1 radioframe structure applicable to frequency division duplex (FDD) and atype-2 radio frame structure applicable to time division duplex (TDD).

FIG. 1(a) illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as transmission time interval (TTI). Forexample, one subframe may be 1 ms in duration and one slot may be 0.5 msin duration. A slot may include a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols in the time domain and includes aplurality of resource blocks (RBs) in the frequency domain. Because the3GPP LTE system adopts OFDMA for downlink, an OFDM symbol represents onesymbol period. An OFDM symbol may be referred to as an SC-FDMA symbol orsymbol period. A Resource Block (RB) is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols included in one slot depends on cyclic prefix(CP) configuration. CP is divided into an extended CP and a normal CP.For example, when OFDM symbols are configured according to normal CP,the number of OFDM symbols included in one slot may be 7. When the OFDMsymbols are configured according to extended CP, the duration of oneOFDM symbol increases and thus the number of OFDM symbols included inone slot is smaller than the number of OFDM symbols included in one slotwhen the OFDM symbols are configured using the normal CP. In theextended CP case, the number of OFDM symbols included in one slot may be6, for example. When a channel status is unstable, for example, when aUE moves at a high speed, the extended CP can be used to reduceinter-symbol interference.

When the normal CP is used, one slot includes 7 OFDM symbols, and thusone subframe includes 14 OFDM symbols. In this case, up to three OFDMsymbols at the start of each subframe can be allocated to a physicaldownlink control channel (PDCCH) and the other three OFDM symbols can beallocated to a physical downlink shared channel (PDSCH).

FIG. 1(b) illustrates the type-2 radio frame structure. The type-2 radioframe includes two half frames each having 5 subframes, a downlink pilottime slot (DwPTS), a guard period (GP), and an uplink pilot time slot(UpPTS). Each subframe includes two slots. The DwPTS is used for initialcell search, synchronization, or channel estimation in a UE, whereas theUpPTS is used for channel estimation in an eNB and uplink transmissionsynchronization in a UE. The GP is a period between a downlink and anuplink, for eliminating interference with the uplink caused bymulti-path delay of a downlink signal. A subframe is composed of twoslots irrespective of radio frame type.

The aforementioned radio frame structure is purely exemplary and thusthe number of subframes included in a radio frame, the number of slotsincluded in a subframe, or the number of symbols included in a slot mayvary.

FIG. 2 illustrates a resource grid for a downlink slot. A downlink slotincludes 7 OFDM symbols in the time domain and an RB includes 12subcarriers in the frequency domain, which does not limit the scope andspirit of the present invention. For example, a slot includes 7 OFDMsymbols in the case of normal CP, whereas a slot includes 6 OFDM symbolsin the case of extended CP. Each element of the resource grid isreferred to as a resource element (RE). An RB includes 12×7 REs. Thenumber of RBs in a downlink slot, N^(DL) depends on a downlinktransmission bandwidth. An uplink slot may have the same structure as adownlink slot.

FIG. 3 illustrates a downlink subframe structure. Up to three OFDMsymbols at the start of the first slot in a downlink subframe are usedfor a control region to which control channels are allocated and theother OFDM symbols of the downlink subframe are used for a data regionto which a PDSCH is allocated. Downlink control channels used in 3GPPLTE include a physical control format indicator channel (PCFICH), aphysical downlink control channel (PDCCH), and a physical hybridautomatic repeat request (ARQ) indicator channel (PHICH). The PCFICH islocated in the first OFDM symbol of a subframe, carrying informationabout the number of OFDM symbols used for transmission of controlchannels in the subframe. The PHICH delivers a HARQacknowledgment/negative acknowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI includes uplinkresource allocation information, downlink resource allocationinformation or an uplink transmit (Tx) power control command for anarbitrary UE group. The PDCCH delivers information about resourceallocation and a transport format for a Downlink Shared Channel(DL-SCH), resource allocation information about an Uplink Shared Channel(UL-SCH), paging information of a Paging Channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a Random Access Responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, Voice Over Internet Protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregationof one or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE corresponds to a plurality ofREs. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a Cyclic RedundancyCheck (CRC) to control information. The CRC is masked by an Identifier(ID) known as a Radio Network Temporary Identifier (RNTI) according tothe owner or usage of the PDCCH. If the PDCCH is directed to a specificUE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE. If thePDCCH carries a paging message, the CRC of the PDCCH may be masked by aPaging Indicator Identifier (P-RNTI). If the PDCCH carries systeminformation, particularly, a System Information Block (SIB), its CRC maybe masked by a system information ID and a System Information RNTI(SI-RNTI). To indicate that the PDCCH carries a Random Access Responsein response to a Random Access Preamble transmitted by a UE, its CRC maybe masked by a Random Access-RNTI (RA-RNTI).

FIG. 4 illustrates an uplink subframe structure. An uplink subframe maybe divided into a control region and a data region in the frequencydomain. A physical uplink control channel (PUCCH) carrying uplinkcontrol information is allocated to the control region and a physicaluplink shared channel (PUSCH) carrying user data is allocated to thedata region. To maintain single carrier property, a UE does not transmita PUSCH and a PUCCH simultaneously. A PUCCH for a UE is allocated to anRB pair in a subframe. The RBs of the RB pair occupy differentsubcarriers in two slots. Thus it is said that the RB pair allocated tothe PUCCH is frequency-hopped over a slot boundary.

MIMO System Modeling

FIG. 5 illustrates the configuration of a communication system includingmultiple antennas.

Referring to FIG. 5(a), when both the number of Tx antennas and thenumber of Rx antennas respectively to N_(T) and N_(R), a theoreticalchannel transmission capacity is increased, compared to use of aplurality of antennas at only one of a transmitter and a receiver. Thechannel transmission capacity is increased in proportion to the numberof antennas. Therefore, transmission rate and frequency efficiency canbe increased remarkably. Given a maximum transmission rate R_(o) thatmay be achieved with a single antenna, the transmission rate may beincreased, in theory, to the product of R_(o) and a transmission rateincrease rate R_(i) illustrated in Equation 1 due to an increase inchannel transmission capacity in case of multiple antennas.R₁=min(N_(T),N_(R))  [Equation 1]

For instance, a MIMO communication system with 4 Tx antennas and 4 Rxantennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. The theoreticalincrease in transmission rate of MIMO communication was demonstrated inthe mid-1990s, various technologies for improving data rate have beenactively studied since then and are now employed in various wirelesscommunication standards such as 3^(rd) generation mobile communicationand next-generation wireless LAN.

A variety of research such as information theory research related tocalculation of multi-antenna throughput in various channel environmentsand multiple access environments, research on radio channel measurementand model derivation in MIMO systems and research on time spatial signalprocessing technology for improvement of transmission reliability anddata rate are underway.

Communication in a MIMO system will be described in detail throughmathematical modeling. It is assumed that N_(T) Tx antennas and N_(R) Rxantennas are present.

Regarding a transmission signal, up to N_(T) pieces of information canbe transmitted through the N_(T) Tx antennas, as expressed as thefollowing vector.s=[s₁,s₂, . . . s_(N) _(T) ]^(T)  [Equation 2]

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let thetransmission power levels of the transmission information be denoted byP₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmissionpower-controlled transmission information vector is given as follows.ŝ=[ŝ₁,ŝ₂, . . . ,ŝ_(N) _(T) ]^(T)=[Ps₁,Ps₂, . . . ,Ps_(N) _(T)]^(T)  [Equation 3]

The transmission power-controlled transmission information ŝ vectors maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & & & 0 \\ & P_{2} & & \\ & & \ddots & \\0 & & & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\ \vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}4} \right\rbrack\end{matrix}$

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) may be generatedby multiplying the transmission power-controlled information vectors ŝby a weight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These N_(T) transmission signals x₁,x₂, . . . , x_(N) _(T) are represented as a vector X, which may bedetermined by Equation 5.

$\begin{matrix}\begin{matrix}{x = \begin{bmatrix}x_{1} \\x_{2} \\ \vdots \\x_{i} \\ \vdots \\x_{N_{T}}\end{bmatrix}} \\{= {\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{12} & w_{12} & \ldots & w_{2N_{T}} \\ \vdots & & \ddots & \\w_{i2} & w_{i2} & \ldots & w_{iN_{T}} \\ \vdots & & \ddots & \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\ \vdots \\{\hat{s}}_{j} \\ \vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}}} \\{= {W\hat{s}}} \\{= {WPs}}\end{matrix} & \left\lbrack {{Equation}5} \right\rbrack\end{matrix}$

Herein, w_(ij) denotes a weight between an i^(th) Tx antenna and aj^(th) piece of information. W is called a weight matrix or a precodingmatrix.

Given N_(R) Rx antennas, signals received at the respective Rx antennas,y₁, y₂, . . . , y_(N) _(T) may be represented as the following vector.y=[y₁,y₂, . . . ,y_(N) _(R) ]^(T)  [Equation 6]

When channels are modeled in the MIMO communication system, they may bedistinguished according to the indexes of Tx and Rx antennas and thechannel between a j^(th) Tx antenna and an i^(th) Rx antenna may berepresented as h_(ij). It is to be noted herein that the index of the Rxantenna precedes that of the Tx antenna in h_(ij).

FIG. 5(b) illustrates channels from N_(T) Tx antennas to an i^(th) Rxantenna. The channels may be represented as vectors and matrices bygrouping them. As illustrated in FIG. 5(b), the channels from the N_(T)Tx antennas to an i^(th) Rx antenna may be expressed as follows.h_(i) ^(T)=[h_(i1),h_(i2), . . . ,h_(iN) _(T) ]  [Equation 7]

Also, all channels from the N_(T) Tx antennas to the N_(R) Rx antennasmay be expressed as the following matrix.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\ \vdots \\h_{i}^{T} \\ \vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{12} & h_{12} & \ldots & h_{2N_{T}} \\ \vdots & & \ddots & \\h_{i2} & h_{i2} & \ldots & h_{{iN}_{T}} \\ \vdots & & \ddots & \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}8} \right\rbrack\end{matrix}$

Actual channels experience the above channel matrix H and then are addedwith Additive White Gaussian Noise (AWGN). The AWGN n₁, n₂, . . . ,n_(N) _(n) added to the N_(R) Rx antennas is given as the followingvector.n=[n₁,n₂, . . . ,n_(N) _(R) ]^(T)  [Equation 9]

From the above modeled equations, the received signal can be expressedas follows.

$\begin{matrix}\begin{matrix}{y = \begin{bmatrix}y_{1} \\y_{2} \\ \vdots \\y_{i} \\ \vdots \\y_{N_{R}}\end{bmatrix}} \\{= {{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{12} & h_{12} & \ldots & h_{2N_{T}} \\ \vdots & & \ddots & \\h_{i2} & h_{i2} & \ldots & h_{{iN}_{T}} \\ \vdots & & \ddots & \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\ \vdots \\x_{j} \\ \vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\ \vdots \\n_{i} \\ \vdots \\n_{N_{R}}\end{bmatrix}}} \\{= {{Hx} + n}}\end{matrix} & \left\lbrack {{Equation}10} \right\rbrack\end{matrix}$

In the meantime, the numbers of rows and columns in the channel matrix Hrepresenting channel states are determined according to the numbers ofTx and Rx antennas. The number of rows is identical to that of Rxantennas, N_(R) and the number of columns is identical to that of Txantennas, N_(T). Thus, the channel matrix H is of size N_(R)×N_(T).

In general, the rank of a matrix is defined as the smaller between thenumbers of independent rows and columns. Accordingly, the rank of thematrix is not larger than the number of rows or columns. The rank of thematrix H, rank (H) is limited as follows.rank(H)≥min(N_(T),N_(R))  [Equation 11]

The rank of a matrix may be defined as the number of non-zero Eigenvalues when the matrix is Eigen-value-decomposed. Similarly, the rank ofa matrix may be defined as the number of non-zero singular values whenthe matrix is singular-value-decomposed. Accordingly, the physicalmeaning of the rank of a channel matrix can be a maximum number ofchannels through which different pieces of information can betransmitted.

In the specification, ‘rank’ with respect to MIMO transmissionrepresents the number of paths through which signals can beindependently transmitted in a specific frequency resource at a specificinstance and ‘number of layers’ refers to the number of signal streamstransmitted through each path. Since a transmitter transmits as manylayers as the number of ranks used for signal transmission, the rankcorresponds to the number of layers unless otherwise mentioned.

Reference Signal (RS)

In a wireless communication system, since packets are transmittedthrough a radio channel, a signal may be distorted during transmission.In order to enable a reception side to correctly receive the distortedsignal, distortion of the received signal should be corrected usingchannel information. In order to detect the channel information, amethod of transmitting a signal, of which both the transmission side andthe reception side are aware, and detecting channel information using adistortion degree when the signal is received through a channel ismainly used. The above signal is referred to as a pilot signal or areference signal (RS).

When transmitting and receiving data using multiple antennas, thechannel states between the transmission antennas and the receptionantennas should be detected in order to correctly receive the signal.Accordingly, each transmission antenna has an individual RS. In moredetail, an independent RS should be transmitted through each Tx port.

A downlink RS includes a Common RS (CRS) shared among all UEs in a celland a Dedicated RS (DRS) for only a specific-UE. It is possible toprovide information for channel estimation and demodulation using suchRSs.

The reception side (UE) estimates the channel state from the CRS andfeeds back an indicator associated with channel quality, such as aChannel Quality Indicator (CQI), a Precoding Matrix Index (PMI) and/or aRank Indicator (RI), to the transmission side (eNodeB). The CRS may bealso called a cell-specific RS. Alternatively, an RS associated with thefeedback of Channel State Information (CSI) such as CQI/PMI/RI may beseparately defined as a CSI-RS.

The DRS may be transmitted through REs if data demodulation on a PDSCHis necessary. The UE may receive the presence/absence of the DRS from ahigher layer and receive information indicating that the DRS is validonly when the PDSCH is mapped. The DRS may be also called a UE-specificRS or a Demodulation RS (DMRS).

FIG. 6 is a diagram showing a pattern of CRSs and DRSs mapped on adownlink RB pair defined in the existing 3GPP LTE system (e.g.,Release-8). The downlink RB pair as a mapping unit of the RSs may beexpressed in units of one subframe on a time domain×12 subcarriers on afrequency domain. That is, on the time axis, one RB pair has a length of14 OFDM symbols in case of the normal CP (FIG. 6(a)) and has a length of12 OFDM symbols in case of the extended CP (FIG. 6(b)).

FIG. 6 shows the locations of the RSs on the RB pair in the system inwhich the eNodeB supports four transmission antennas. In FIG. 7 ,Resource Elements (REs) denoted by “0”, “1”, “2” and “3” indicate thelocations of the CRSs of the antenna port indexes 0, 1, 2 and 3,respectively. In FIG. 6 , the RE denoted by “D” indicates the locationof the DRS.

Hereinafter, the CRS will be described in detail.

The CRS is used to estimate the channel of a physical antenna and isdistributed over the entire band as an RS which is able to be commonlyreceived by all UEs located within a cell. The CRS may be used for CSIacquisition and data demodulation.

The CRS is defined in various formats according to the antennaconfiguration of the transmission side (eNodeB). The 3GPP LTE (e.g.,Release-8) system supports various antenna configurations, and adownlink signal transmission side (eNodeB) has three antennaconfigurations such as a single antenna, two transmission antennas andfour transmission antennas. If the eNodeB performs single-antennatransmission, RSs for a single antenna port are arranged. If the eNodeBperforms two-antenna transmission, RSs for two antenna ports arearranged using a Time Division Multiplexing (TDM) and/or FrequencyDivision Multiplexing (FDM) scheme. That is, the RSs for the two antennaports are arranged in different time resources and/or differentfrequency resources so as to be distinguished from each other. Inaddition, if the eNodeB performs four-antenna transmission, RSs for fourantenna ports are arranged using the TDM/FDM scheme. The channelinformation estimated by the downlink signal reception side (UE) throughthe CRSs may be used to demodulate data transmitted using a transmissionscheme such as single antenna transmission, transmit diversity,closed-loop spatial multiplexing, open-loop spatial multiplexing, orMulti-User MIMO (MU-MIMO).

If multiple antennas are supported, when RSs are transmitted from acertain antenna port, the RSs are transmitted at the locations of theREs specified according to the RS pattern and any signal is nottransmitted at the locations of the REs specified for another antennaport.

The rule of mapping the CRSs to the RBs is defined by Equation 12.

$\begin{matrix}{k = {{6m} + {\left( {v + v_{shift}} \right){mod}6}}} & \left\lbrack {{Equation}12} \right\rbrack\end{matrix}$ $l = \left\{ \begin{matrix}{0,{N_{symb}^{DL} - 3}} & {{{if}p} \in \left\{ {0,1} \right\}} \\1 & {{{if}p} \in \left\{ {2,3} \right\}}\end{matrix} \right.$ m = 0, 1, …, 2 ⋅ N_(RB)^(DL) − 1m^(′) = m + N_(RB)^(max, DL) − N_(RB)^(DL) $\begin{matrix}{v = \left\{ \begin{matrix}0 & {{{if}p} = {{0{and}l} = 0}} \\3 & {{{if}p} = {{0{and}l} \neq 0}} \\3 & {{{if}p} = {{1{and}l} = 0}} \\0 & {{{if}p} = {{1{and}l} \neq 0}} \\{3\left( {n_{s}{mod}2} \right)} & {{{if}p} = 2} \\{3 + {3\left( {n_{s}{mod}2} \right)}} & {{{if}p} = 3}\end{matrix} \right.} & \end{matrix}$

In Equation 12, k denotes a subcarrider index, 1 denotes a symbol index,and p denotes an antenna port index. N_(symb) ^(DL) denotes the numberof OFDM symbols of one downlink slot, N_(RB) ^(DL) denotes the number ofRBs allocated to the downlink, n_(s) denotes a slot index, and N_(ID)^(cell) denotes a cell ID. mod indicates a modulo operation. Thelocation of the RS in the frequency domain depends on a value V_(shift).Since the value V_(shif) depends on the cell ID, the location of the RShas a frequency shift value which varies according to the cell.

More specifically, in order to increase channel estimation performancethrough the CRSs, the locations of the CRSs in the frequency domain maybe shifted so as to be changed according to the cells. For example, ifthe RSs are located at an interval of three subcarriers, the RSs arearranged on 3k-th subcarriers in one cell and arranged on (3k+1)-thsubcarriers in the other cell. In view of one antenna port, the RSs arearranged at an interval of 6 REs (that is, interval of 6 subcarriers) inthe frequency domain and are separated from REs, on which RSs allocatedto another antenna port are arranged, by 3 REs in the frequency domain.

In addition, power boosting is applied to the CRSs. The power boostingindicates that the RSs are transmitted using higher power by bringing(stealing) the powers of the REs except for the REs allocated for theRSs among the REs of one OFDM symbol.

In the time domain, the RSs are arranged from a symbol index (l=0) ofeach slot as a starting point at a constant interval. The time intervalis differently defined according to the CP length. The RSs are locatedon symbol indexes 0 and 4 of the slot in case of the normal CP and arelocated on symbol indexes 0 and 3 of the slot in case of the extendedCP. Only RSs for a maximum of two antenna ports are defined in one OFDMsymbol. Accordingly, upon four-transmission antenna transmission, theRSs for the antenna ports 0 and 1 are located on the symbol indexes 0and 4 (the symbol indexes 0 and 3 in case of the extended CP) of theslot and the RSs for the antenna ports 2 and 3 are located on the symbolindex 1 of the slot. The frequency locations of the RSs for the antennaports 2 and 3 in the frequency domain are exchanged with each other in asecond slot.

Hereinafter, the DRS will be described in detail.

The DRS (or the UE-specific RS) is used to demodulate data. A precodingweight used for a specific UE upon multi-antenna transmission is alsoused in an RS without change so as to estimate an equivalent channel, inwhich a transfer channel and the precoding weight transmitted from eachtransmission antenna are combined, when the UE receives the RSs.

The existing 3GPP LTE system (e.g., Release-8) supportsfour-transmission antenna transmission as a maximum and the DRS for Rank1 beamforming is defined. The DRS for Rank 1 beamforming is also denotedby the RS for the antenna port index 5. The rule of the DRS mapped onthe RBs is defined by Equations 13 and 14. Equation 13 is for the normalCP and Equation 14 is for the extended CP.

$\begin{matrix}{k = {{\left( k^{\prime} \right){mod}N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}} & \left\lbrack {{Equation}13} \right\rbrack\end{matrix}$ $k^{\prime} = \left\{ \begin{matrix}{{4m^{\prime}} + v_{shift}} & {{{if}l} \in \left\{ {2,3} \right\}} \\{{4m^{\prime}} + {\left( {2 + v_{shift}} \right){mod}4}} & {{{if}l} \in \left\{ {5,6} \right\}}\end{matrix} \right.$ $l = \left\{ \begin{matrix}3 & {l^{\prime} = 0} \\6 & {l^{\prime} = 1} \\2 & {l^{\prime} = 2} \\5 & {l^{\prime} = 3}\end{matrix} \right.$ $l^{\prime} = \left\{ \begin{matrix}{0,1} & {{{if}n_{s}{mod}2} = 0} \\{2,3} & {{{if}n_{s}{mod}2} = 1}\end{matrix} \right.$ m^(′) = 0, 1, …, 3N_(RB)^(PDSCH) − 1v_(shift) = N_(ID)^(cell)mod3 $\begin{matrix}{k = {{\left( k^{\prime} \right){mod}N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}} & \left\lbrack {{Equation}14} \right\rbrack\end{matrix}$ $k^{\prime} = \left\{ \begin{matrix}{{3m^{\prime}} + v_{shift}} & {{{if}l} = 4} \\{{3m^{\prime}} + {\left( {2 + v_{shift}} \right){mod}3}} & {{{if}l} = 1}\end{matrix} \right.$ $l = \left\{ \begin{matrix}4 & {l^{\prime} \in \left\{ {0,2} \right\}} \\1 & {l^{\prime} = 1}\end{matrix} \right.$ $l^{\prime} = \left\{ \begin{matrix}0 & {{{if}n_{s}{mod}2} = 0} \\{1,2} & {{{if}n_{s}{mod}2} = 1}\end{matrix} \right.$ m^(′) = 0, 1, …, 4N_(RB)^(PDSCH) − 1v_(shift) = N_(ID)^(cell)mod3

In Equations 13 and 14, k denotes a subcarrider index, 1 denotes asymbol index, and p denotes an antenna port index. N_(SC) ^(RB) denotesthe resource block size in the frequency domain and is expressed by thenumber of subcarriers. n_(PRB) denotes a physical resource block number.N_(RB) ^(PDSCH) denotes the bandwidth of the RB of the PDSCHtransmission. n_(s) denotes a slot index, and N_(ID) ^(cell) denotes acell ID. mod indicates a modulo operation. The location of the RS in thefrequency domain depends on a value V_(shift). Since the value V_(shif)depends on the cell ID, the location of the RS has a frequency shiftvalue which varies according to the cell.

In order to support spectral efficiency higher than that of the existing3GPP LTE (e.g., Release-8) system, a system (e.g., an LTE-A system)having the extended antenna configuration may be designed. The extendedantenna configuration may have, for example, eight transmissionantennas. In the system having the extended antenna configuration, UEswhich operate in the existing antenna configuration needs to besupported, that is, backward compatibility needs to be supported.Accordingly, it is necessary to support a RS pattern according to theexisting antenna configuration and to design a new RS pattern for anadditional antenna configuration.

Downlink RSs in LTE have been defined only for up to 4 antenna ports.Therefore, when an LTE-A system has 4 to 8 downlink transmit antennas,there is a need to additionally define RSs for the antenna ports in theLTE-A system. Both an RS for channel measurement and an RS for datademodulation need to be taken into consideration as RSs for up to 8transmit (Tx) antenna ports.

If RSs for up to 8 transmit antennas are added to time-frequency domainsin which a CRS defined in the LTE standard is transmitted every subframeover an entire band, RS overhead is excessively increased from theviewpoint of RS transmission. Therefore, RS overhead reduction should beconsidered when designing a new RS of a maximum of 8 antenna ports.

RSs newly introduced in the LTE-A system may be largely classified intotwo types. One is a Channel State Information RS (CSI-RS) which is an RSfor channel measurement for calculation/selection of an RI, a PrecodingMatrix Index (PMI), a CQI, or the like. The other is a DeModulation RS(DM RS) (or UE-specific RS) which is an RS for demodulating datatransmitted through up to 8 transmit antennas.

CSI-RS for channel measurement is characterized in that the CSI-RS isdesigned mainly for channel measurement unlike the CRS of theconventional LTE system which is used not only for measurement ofhandover or the like but also for data modulation. Of course, the CSI-RSmay also be used for measurement of handover or the like. Since theCSI-RS is transmitted only for the purpose of obtaining informationregarding channel conditions, the CSI-RS need not be transmitted everysubframe, unlike the CRS of the conventional LTE system. Accordingly, toreduce CSI-RS overhead, the CSI-RS may be designed to be transmittedintermittently (periodically) in the time axis.

FIG. 7 is a diagram showing exemplary CSI-RS patterns. In more detail,in case of one RB pair (in case of a normal CP, 14 OFDM symbols in atime domain×12 subcarriers in a frequency domain) used for DL datatransmission, FIG. 7 shows the locations of resource elements (REs) usedfor CSI-RS transmission. One CSI-RS pattern shown in FIGS. 7(a) to 7(e)may be used in a certain DL subframe. CSI-RS may be transmitted to 8antenna ports (Antenna Port Indexes #15 to #22) additionally defined inthe LTE-A system. CSI-RSs for different antenna ports are located atdifferent frequency resources (subcarriers) and/or different timeresources (OFDM symbols), such that each CSI-RS cam be identified. Thatis, CSI-RSs may be multiplexed according to the FDM and/or TDMscheme(s). In addition, CSI-RSs of different antenna ports located atthe same time-frequency resources may be identified from each other bydifferent orthogonal codes (that is, the CSI-RSs may be multiplexedaccording to the CDM scheme). As can be seen from FIG. 7(a), CSI-RSs ofAntenna Ports #15 and #16 may be located at REs denoted by CSI-RS CDMGroup #1, and may be multiplexed by orthogonal codes. CSI-RSs of AntennaPorts #17 and #18 may be located at REs denoted by CSI-RS CDM Group #2as shown in FIG. 7(a), and may be multiplexed by orthogonal codes. InFIG. 7(a), CSI-RSs of Antenna Ports #19 and #20 may be located at REsdenoted by CSI-RS CDM Group #3, and may be multiplexed by orthogonalcodes. CSI-RSs of Antenna Ports #21 and #22 may be located at REsdenoted by CSI-RS CDM Group #4 as shown in FIG. 7(a), and may bemultiplexed by orthogonal codes. The same principles described in FIG.7(a) may be applied to FIGS. 7(b) to 7(e).

As described above, when data is transmitted in a certain DL subframe,(dedicated) DM RS is transmitted to a UE in which data transmission isscheduled. DM RS dedicated for a specific UE may be designed only in aresource region (a time-frequency domain in which data of thecorresponding UE is transmitted) in which the corresponding UE isscheduled. In LTE-A, a high-order MIMO, multi-cell transmission, andevolved MU-MIMO, etc. have been considered. To support the efficient RSmanagement and the evolved Tx scheme, DM RS—based data modulation hasbeen considered in LTE-A. That is, unlike DMRS (Antenna Port Index #5)(See FIG. 6(d)) for Rank #1 beamforming defined in legacy 3GPP LTE(e.g., Release-8), DMRS for two or more layers may be defined to supportdata transmission through additional antennas.

FIG. 8 is a diagram showing exemplary DMRS patterns defined in LTE-A. InFIG. 8 , DMRS patterns may indicate RE positions to which DMRS is mappedwithin one RB pair (=14 OFDM symbols×12 subcarriers) of a normal CPsubframe.

Referring to FIG. 8 , if Rank of PDSCH is set to 1 or 2, a total of 12REs (each of which is denoted by ‘L’ in FIG. 8 ) of one RB pair may beused for DMRS transmission. DMRS of Layer #1 and DMRS of Layer #2 may beCDM-processed by orthogonal codes corresponding to ‘Spreading Factor=2’.In more detail, DMRS of Layer #1 and DMRS of Layer #2 may be spread on atime axis, and a total of 4 REs is used for DMRS transmission on asingle subcarrier, such that DMRSs for Layers #1 and #2 are repeated ata slot boundary. That is, DMRS for Layer #1 and DMRS for Layer #2 aremapped to the same RE, and DMRS for Layer #1 and DMRS for Layer #2 maybe distinguished from each other by an orthogonal code (OC) multipliedby a time domain (covering OFDM symbols).

Referring to FIG. 8 , if Rank of PDSCH is set to 3 or higher, 12 REs(each of which is denoted by ‘H’ in FIG. 7 ) are additionally used forDMRS transmission. DMRS for Layer #3 and DMRS for Layer #4 may beFDM-processed with DMRSs for Layers #1 and #2. That is, subcarriers towhich DMRSs for Layers #3 and #4 are mapped are different fromsubcarriers to which Layers #1 and #2 are mapped. In addition, DMRS forLayer #3 and DMRS for Layer #4 may be CDM-processed by an orthogonalcode (OC) corresponding to ‘Spreading Factor=2’ in a time domain.

In case of ‘Rank=5 or higher’, additional REs other than REs used for‘Rank=1, 2, 3, or 4’ are not used for DMRSs of Layers #5, #6, #7, and#8. However, REs to which DMRSs of Layers #1, #2, #3, and #4 are mappedmay be reused, and OC having ‘Spreading Factor-4’ is multiplied by DMRSsfor Layers #5, #6, #7, and #8 on a time axis, such that the DMRSs forLayers #5, #6, #7, and #8 may be distinguished from DMRSs for Layers #1,#2, #3, and #4 according to the CDM scheme. For example, DMRSs forLayers #5 and #7 may be mapped to REs identical to REs (each of which isdenoted by ‘L’ in FIG. 8 ) to which DMRSs for Layers #1 and #2 aremapped, and DMRSs for Layers #6 and #8 may be mapped to REs identical toREs (each of which is denoted by ‘H’ in FIG. 8 ) to which DMRSs forLayers #3 and #4 are mapped. In this case, DMRSs for Layers #1, #2, #5,and #7 may be identified from each other in the time domain according tothe CDM scheme, and DMRSs for Layers #3, #4, #6, and #8 may beidentified from each other in the time domain according to the CDMscheme. DMRSs for Layers #1, #2, #5, and #7 may also be distinguishedfrom DMRSs for Layers #3, #4, #6, and #8 according to the FDM scheme.

For example, DMRS for Layer #1 may be spread on orthogonal codes[+1+1+1+1] corresponding to ‘Spreading Factor-4’ on four REs of a singlesubcarrier. In order to maintain orthogonality with DMRS for Layer #1,DMRS for Layer #5 may be spread on orthogonal codes [+1+1−1−1]corresponding to ‘Spreading Factor-4’. In other words, orthogonal codesapplied to DMRSs for Layers #5, #6, #7, and #8 are designed to use thesame REs as in DMRSs for Layers #1, #2, #3, and #4 as well as tomaintain orthogonality, and codes (obtained by phase inversion) adjustedto make a phase difference of 180° at a slot boundary (between a firstslot and a second slot) may be used under the condition that OC of‘Spreading Factor=2’ is used.

The following Table 1 shows spreading codes applied to theabove-mentioned DMRS patterns.

TABLE 1 Antenna port p [w _(p)(0) w _(p)(1) w _(p)(2) w _(p)(3)] 7 [+1+1 +1 +1] 8 [+1 −1 +1 −1] 9 [+1 +1 +1 +1] 10 [+1 −1 +1 −1] 11 [+1 +1 −1−1] 12 [−1 −1 +1 +1] 13 [+1 −1 −1 +1] 14 [−1 +1 +1 −1]

In Table 1, Antenna Ports (p) #7 to #14 may respectively indicatelogical antennas (i.e., antenna ports) through which DMRSs for PDSCH Txlayers #1 to #8 are transmitted. DMRSs for Antenna Ports #7, #8, #11,and #13 may be mapped to the same 12 REs (each of which is denoted by‘L’ in FIG. 8 ), and DMRSs for Antenna Ports #9, #10, #12, and #14 maybe mapped to the same 12 REs (each of which is denoted by ‘H’ in FIG. 8).

Coordinated Multi-Point: CoMP

CoMP transmission/reception scheme (which is also referred to asco-MIMO, collaborative MIMO or network MIMO) is proposed to meetenhanced system performance requirements of 3GPP LTE-A. CoMP can improvethe performance of a UE located at a cell edge and increase averagesector throughput.

In a multi-cell environment having a frequency reuse factor of 1, theperformance of a UE located at a cell edge and average sector throughputmay decrease due to inter-cell interference (ICI). To reduce ICI, aconventional LTE system uses a method for allowing a UE located at acell edge in an interfered environment to have appropriate throughputusing a simple passive scheme such as fractional frequency reuse (FFR)through UE-specific power control. However, it may be more preferable toreduce ICI or reuse ICI as a signal that a UE desires rather thandecreasing frequency resource use per cell. To achieve this, CoMP can beapplied.

CoMP applicable to downlink can be classified into joint processing (JP)and coordinated scheduling/beamforming (CS/CB).

According to the JP, each point (eNB) of a CoMP coordination unit canuse data. The CoMP coordination unit refers to a set of eNBs used for acoordinated transmission scheme. The JP can be divided into jointtransmission and dynamic cell selection.

The joint transmission refers to a scheme through which PDSCHs aresimultaneously transmitted from a plurality of points (some or all CoMPcoordination units). That is, data can be transmitted to a single UEfrom a plurality of transmission points. According to jointtransmission, quality of a received signal can be improved coherently ornon-coherently and interference on other UEs can be actively erased.

Dynamic cell selection refers to a scheme by which a PDSCH istransmitted from one point (in a CoMP coordination unit). That is, datais transmitted to a single UE from a single point at a specific time,other points in the coordination unit do not transmit data to the UE atthe time, and the point that transmits the data to the UE can bedynamically selected.

According to the CS/CB scheme, CoMP coordination units cancollaboratively perform beamforming of data transmission to a single UE.Here, user scheduling/beaming can be determined according tocoordination of cells in a corresponding CoMP coordination unit althoughdata is transmitted only from a serving cell.

In case of uplink, coordinated multi-point reception refers to receptionof a signal transmitted according to coordination of a plurality ofpoints geographically spaced apart from one another. A CoMP receptionscheme applicable to uplink can be classified into joint reception (JR)and coordinated scheduling/beamforming (CS/CB).

JR is a scheme by which a plurality of reception points receives asignal transmitted over a PUSCH and CS/CB is a scheme by which userscheduling/beamforming is determined according to coordination of cellsin a corresponding CoMP coordination unit while one point receives aPUSCH.

A UE can receive data from multi-cell base stations collaborativelyusing the CoMP system. The base stations can simultaneously support oneor more UEs using the same radio frequency resource, improving systemperformance. Furthermore, a base station may perform space divisionmultiple access (SDMA) on the basis of CSI between the base station anda UE.

In the CoMP system, a serving eNB and one or more collaborative eNBs areconnected to a scheduler through a backbone network. The scheduler canoperate by receiving channel information about a channel state betweeneach UE and each collaborative eNB, measured by each eNB, through thebackbone network. For example, the scheduler can schedule informationfor collaborative MIMO operation for the serving eNB and one or morecollaborative eNBs. That is, the scheduler can directly directcollaborative MIMO operation to each eNB.

As described above, the CoMP system can be regarded as a virtual MIMOsystem using a group of a plurality of cells. Basically, a communicationscheme of MIMO using multiple antennas can be applied to CoMP.

Downlink Channel State Information (CSI) Feedback

MIMO can be categorized into an open-loop scheme and a closed-loopscheme. The open-loop scheme performs MIMO transmission at a transmitterwithout feedback of CSI from a MIMO receiver, whereas the closed-loopscheme performs MIMO transmission at the transmitter using feedback ofCSI from the MIMO receiver. In closed-loop MIMO, each of the transmitterand the receiver can perform beamforming based on CSI to obtain MIMO Txantenna multiplexing gain. The transmitter (e.g. eNB) can allocate anuplink control channel or an uplink shared channel to the receiver (e.g.UE) such that the receiver can feed back CSI.

CSI fed back may include a rank indicator (RI), a precoding matrix index(PMI) and a channel quality indictor (CQI).

The RI indicates information about a channel rank. The channel rankrepresents a maximum number of layers (or streams) through whichdifferent pieces of information can be transmitted through the sametime-frequency resource. The RI is determined by long term fading of achannel, and thus the RI can be fed back to an eNB at a longer periodthan the PMI and CQI.

The PMI is information about a precoding matrix used for transmissionfrom a transmitter and is a value in which spatial characteristics of achannel are reflected. Precoding refers to mapping a transport layer toa transmit antenna. A layer-to-antenna mapping relation can bedetermined by a precoding matrix. The PMI indicates a precoding matrixindex of an eNB preferred by a UE based on a metric such assignal-interference plus noise ratio (SINR). To reduce feedback overheadof precoding information, the transmitter and receiver can share acodebook including precoding matrices and only an index indicating aspecific precoding matrix in the codebook can be fed back.

The CQI indicates channel quality or channel intensity. The CQI can berepresented as a predetermined MCS combination. That is, a fed back CQIindex indicates a corresponding modulation scheme and a code rate. TheCQI represents a value in which a reception SINR that can be obtainedwhen an eNB configures a spatial channel using the PMI is reflected.

In a system supporting an extended antenna configuration (e.g. LTE-A),additional multi-user diversity is obtained using multi-user MIMO(MU-MIMO). When an eNB performs downlink transmission using CSI fed backby one of multiple UEs, it is necessary to prevent downlink transmissionfrom interfering with other UEs since an interference channel is presentbetween UEs multiplexed in the antenna domain in MU-MIMO. Accordingly,MU-MIMO requires more accurate CSI feedback than single user MIMO(SU-MIMO).

A new CSI feedback scheme that improves CSI composed of the RI, PMI andCQI can be applied in order to measure and report more accurate CSI. Forexample, precoding information fed back by a receiver can be indicatedby a combination of two PMIs. One (first PMI) of the two PMIs is longterm and/or wideband information and may be denoted as W1. The other PMI(second PMI) is short term and/or subband information and may be denotedas W1. A final PMI can be determined by a combination (or function) ofW1 and W2. For example, if the final PMI is W, W can be defined asW=W1*W2 or W=W2*W1.

Here, W1 reflects frequency and/or temporal average characteristics of achannel. In other words, W1 can be defined as CSI reflectingcharacteristics of a long-term channel in the time domain,characteristics of a wideband channel in the frequency domain orcharacteristics of a long-term and wide-band channel. To simplyrepresent these characteristics of W1, W1 is referred to as long termwideband CSI (or long term wideband PMI) in this specification.

W2 reflects instantaneous channel characteristics compared to W1. Inother words, W2 can be defined as CSI reflecting characteristics of ashort-term channel in the time domain, characteristics of a subbandchannel in the frequency domain or characteristics of a short-term andsubband channel. To simply represent these characteristics of W2, W2 isreferred to as short term subband CSI (or short term subband PMI) inthis specification.

To determine a final precoding matrix W from two different pieces ofinformation (e.g. W1 and W2) representing channel states, it isnecessary to configure separate codebooks (i.e. a first codebook for W1and a second codebook for W2) composed of precoding matricesrepresenting the information. A codebook configured in this manner maybe called a hierarchical codebook. Determination of a final codebookusing the hierarchical codebook is called hierarchical codebooktransformation.

A codebook can be transformed using a long-term covariance matrix of achannel, represented by Equation 15, as exemplary hierarchical codebooktransformation.W=norm(W1 W2) [Equation 15]

In Equation 15, W1 (long term wideband PMI) denotes an element (i.e.codeword) constituting a codebook (e.g. first codebook) generated toreflect long term wideband channel information. That is, W1 correspondsto a precoding matrix included in the first codebook that reflects thelong term wideband channel information. W2 (short term subband PMI)represents a codeword constituting a codebook (e.g. second codebook)generated to reflect short term/subband channel information. That is, W2corresponds to a precoding matrix included in the second codebook thatreflects the short term subband channel information. W is a codeword ofa transformed final codebook and norm(A) denotes a matrix in which thenorm of each column of matrix A is normalized to 1.

W1 and W2 may have structures as represented by Equation 16.

$\begin{matrix}{{{W1}(i)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}} & \left\lbrack {{Equation}16} \right\rbrack\end{matrix}$${W2(j)} = {\overset{r{columns}}{\overset{︷}{\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & & e_{M}^{m} \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \ldots & {\gamma_{j}e_{M}^{m}}\end{bmatrix}}}\left( {{{if}{rank}} = r} \right)}$

In Equation 16, W1 can be defined as a block diagonal matrix and blockscorrespond to the same matrix X_(i). A block X_(i) can be defined as a(Nt/2)×M matrix. Here, Nt denotes the number of Tx antennas. e_(M) ^(p)(p=k, l, . . . , m) is an M×1 vector wherein a p-th element of M vectorelements represents 1 and other elements represent 0. When W1 ismultiplied by e_(M) ^(p), a p-th column is selected from columns of W1and thus this vector can be called a selection vector. The number ofvectors fed back at a time to represent a long term wideband channelincreases as M increases, to thereby improve feedback accuracy. However,the codebook size of W1 fed back with low frequency decreases and thecodebook size of W2 fed back with high frequency increases as Mincreases, increasing feedback overhead. That is, there is a tradeoffbetween feedback overhead and feedback accuracy. Accordingly, M can bedetermined such that feedback overhead is not excessively increased andappropriate feedback accuracy is maintained. As to W2, α_(j), β_(j) andare predetermined phase values. In Equation 16, 1≤k,l,m≤M and k, l and mare integers.

The codebook structure represented by Equation 16 uses a cross polarizedantenna configuration and reflects correlation characteristics of achannel, generated when antenna spacing is narrow (when a distancebetween neighboring antennas is less than half a signal wavelength). Forexample, cross polarized antenna configurations may be represented asshown in

Table 2.

TABLE 2 2Tx cross- polarized antenna configuration

4Tx cross- polarized antenna configuration

8Tx cross- polarized antenna configuration

In Table 2, an 8Tx cross polarized antenna configuration is composed oftwo antenna groups having orthogonal polarizations. Antennas belongingto antenna group 1 (antennas 1, 2, 3 and 4) may have the samepolarization (e.g. vertical polarization) and antennas belonging toantenna group 2 (antennas 5, 6 7 and 8) may have the same polarization(e.g. horizontal polarization). The two antenna groups are co-located.For example, antennas 1 and 5 can be co-located, antennas 2 and 6 can beco-located, antennas 3 and 7 can be co-located and antennas 4 and 8 canbe co-located. In other words, antennas in an antenna group have thesame polarization as in a uniform linear array (ULA) and a correlationbetween antennas in an antenna group has a linear phase incrementcharacteristic. Furthermore, a correlation between antenna groups has aphase rotation characteristic.

Since a codebook is composed of values obtained by quantizing a channel,it is necessary to design the codebook by reflecting actual channelcharacteristics therein. To describe reflection of actual channelcharacteristics in codewords of a codebook designed as represented byEquation 16, a rank-1 codebook is exemplified. Equation 17 representsdetermination of a final codeword W by multiplying codeword W1 bycodeword W2 in the case of rank 1.

$\begin{matrix}{{W1(i)*W2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}17} \right\rbrack\end{matrix}$

In Equation 17, the final codeword is represented by a vector of Nt×1and is composed of an upper vector X_(i)(k) and a lower vectorα_(j)X_(i)(k) which respectively represent correlations betweenhorizontal antenna groups and vertical antenna groups of cross polarizedantennas. X_(i)(k) is preferably represented as a vector (e.g. DFTmatrix) having linear phase increment in which correlation betweenantennas in each antenna group is reflected.

When the above-described codebook is used, higher channel feedbackaccuracy can be achieved compared to a case in which a single codebookis used. Single-cell MU-MIMO can be performed using high accuracychannel feedback and thus high accuracy channel feedback is necessaryfor CoMP operation. For example, plural eNBs cooperatively transmit thesame data to a specific UE in CoMP JT operation, and thus this systemcan be theoretically regarded as a MIMO system in which plural antennasare geographically distributed. That is, even when MU-MIMO operation isperformed in CoMP JT, high channel information accuracy is necessary toavoid interference between co-scheduled UEs. In addition, CoMP CB alsorequires accurate channel information in order to avoid interference ofa neighboring cell, applied to a serving cell.

Radio Link Monitoring (RLM)

The UE may monitor a DL radio link quality of a serving cell (i.e., aprimary serving cell used when multiple serving cells are configured),and may inform a higher layer of the corresponding link status. If alink quality measured by a lower layer (e.g., a physical layer) is lessthan a predetermined threshold value (e.g., Q_(out)), “out-of-sync”indication may be provided to a higher layer. Conversely, if the linkquality is higher than a predetermined threshold (e.g., “in-sync”indication may be transferred from a lower layer to a higher layer.Radio Link Quality may be determined according to whether the PDCCHdecoding error probability (e.g., Block Error Rate (BLER) orSignal-to-Interference plus Noise Ratio (SINR)) satisfies apredetermined reference. If the measured link quality continuously staysin a predetermined reference or less during a predetermined time, itbecomes more difficult to maintain connection to the serving cell anylonger, such that Radio Link Failure (RLF) may be declared.

As described above, RLM may be based on a link quality estimated on thebasis of the PDCCH decoding error probability. Meanwhile, the evolvedwireless communication system has proposed various methods for improvingPDCCH performance. If the legacy RLM is carried out even when a newPDCCH Tx/Rx technique is used, the PDCCH decoding error probability iswrongly estimated so that the actual rank quality may not be incorrectlymonitored. Accordingly, there is a need to modify the RLM technique inconsideration of a new PDCCH Tx/Rx scheme. The present inventionproposes various methods for correctly and efficiently performing RLM inconsideration of a new PDCCH Tx/Rx method.

UE-Specific RS Based PDCCH

Only the open-loop Tx diversity is defined in the legacy wirelesscommunication system such that the open-loop Tx diversity is defined asa MIMO Tx technique applicable to PDCCH. According to the open-loop Txdiversity scheme, the precoding matrix (i.e., the mapping relationshipbetween a layer and an antenna port) is predetermined without feedbackfrom the reception end, so that it is impossible to perform theprecoding operation appropriate for a changed channel state.

On the other hand, a method for employing a UE-specific RS for PDCCH hasbeen discussed in the evolved wireless communication system. TheUE-specific RS is an RS transmitted only to each UE. In addition,precoding is applied to the UE-specific RS, and the same precoding mayalso be applied to data transmission. The precoding matrix applied toUE-specific RS and data transmission may also be determined within a set(or codebook) of predefined precoding matrices. Therefore, the channelestimated on the basis of UE-specific RS by the reception end (i.e.,receiver) may correspond to a channel to which precoding is applied. Ifdata demodulation is carried out using the channel estimation, there isno need to indicate additional precoding information (e.g., specificinformation (or Tx PMI) indicating which one of precoding matrices isused within the predefined codebook) for data demodulation. In otherwords, when channel estimation is performed on the basis of UE-specificRS and data is demodulated on the basis of the estimated channel, it isobvious (transparent) to the UE that which precoding was applied toUE-specific RS and data. That is, when PDCCH is transmitted using theUE-specific RS, there is no need to perform signaling of precodinginformation, and the base station (BS) can apply the precoding operationappropriate for a channel situation to a PDCCH. For example, PDCCHperformance of a low-mobility UE capable of performing a high CSIfeedback can be more improved. That is, the BS determines the precodingappropriate for a channel state fed back from the UE, such that thedetermined precoding may be applied to PDCCH and UE-specific RS. Thisprecoding may be optimal for a channel state obtained from a PDCCHreception time of the UE under the condition that there is a low channelstate variation, so that UE performance for demodulating a PDCCH on thebasis of a channel estimated using the UE-specific RS can be improved.

In case of using the above-mentioned UE-specific RS based PDCCH, astructure more appropriately modified than the legacy channel structuremay be used. For example, a data region (i.e., the remaining OFDM symbolregions other than some initial OFDM symbols within one subframe) otherthan a control region for use in the legacy DL subframe structure shownin FIG. 3 may be used for transmission of a new PDCCH. In order todistinguish a new PDCCH from the legacy PDCCH, the new PDCCH may also bereferred to as Enhanced-PDCCH (E-PDCCH).

FIG. 9 is a diagram showing exemplary E-PDCCH structures. As can be seenfrom FIG. 9 , a time domain (e.g., slots or OFDM symbols) to betransmitted may be differently configured according to E-PDCCH types.For example, E-PDCCH1 may be E-PDCCH carrying DCI related to DLassignment, and may be mapped to a region corresponding to a first slotfrom among the PDSCH region as shown in FIG. 9 . E-PDCCH2 may be definedas E-PDCCH carrying DCI related to UL grant. As shown in FIG. 9 ,E-PDCCH1 may be mapped to a region corresponding to a second slot fromamong the PDSCH region. Alternatively, E-PDCCH3 may be a specificE-PDCCH capable of being mapped to two slots of the PDSCH region withoutdistinction between DL assignment and UL grant. Types and/or resourcelocations of the above E-PDCCH are disclosed for illustrative purposesonly, and the scope or spirit of the present invention is not limitedthereto.

On the contrary, a link quality of RLM may be determined on the basis ofthe PDCCH decoding error probability. If the link quality is estimatedon the basis of E-PDCCH (specifically, UE-specific RS based E-PDCCH),precoding to be applied to E-PDCCH must be reflected to accuratelyperform RLM. However, according to the above UE-specific RS basedE-PDCCH, UE-specific RS and E-PDCCH are equally precoded, and specificinformation indicating which precoding was applied is transparent orapparent to the UE. That is, it is impossible for the UE to recognizewhich precoding was applied to E-PDCCH. In this case, there may ariseobscurity in decoding the E-PDCCH decoding probability for RLM.

Various examples capable of correctly and efficiently performing RLMdepending upon the UE-specific RS based E-PDCCH while simultaneouslyremoving such ambiguity will hereinafter be described in detail. Thecontrol channel (e.g., UE-specific RS based E-PDCCH or other controlchannels) precoded on the basis of the precoding information unknown tothe UE will hereinafter be referred to as “precoding-based controlchannel”. That is, the following examples may correspond to variousmethods for correctly and efficiently performing the RLM based on theprecoding-based control channel.

Embodiment 1

The following embodiments relate to methods for deciding precodinginformation of the precoding-based control channel by the receiver(e.g., UE) of the precoding-based control channel.

Embodiment 1-1

When RLM of the precoding-based control channel is performed, it isassumed that a specific precoding matrix is applied to theprecoding-based control channel, such that the present inventionproposes a method for estimating a link quality on the basis of thisassumption. For example, it is assumed that the UE can recognize whichprecoding matrix is applied to UE-specific RS and/or precoding-basedcontrol channel by recognizing a channel status, etc. estimated fromother reference signals (RSs), such that the UE can perform RLM of theprecoding-based control channel.

For example, the UE can derive precoding information to be used for RLMof the precoding-based control channel from CSI-RS. As described above,the UE informs the UE of CSI-RS configuration, and transmits CSI-RS. TheUE selects a preferred precoding matrix on the basis of CSI_RS, andfeeds back the selected precoding matrix to the BS. In accordance withthe present invention, the UE estimates a channel using CSI-RS, andselects a PMI optimal for the estimated channel status, such that the UEmay operate to estimate a link quality based on the assumption that thecorresponding PMI is used. In this case, the optimum PMI may correspondto a first preferred precoding matrix (in which the highest SINR isexpected) in a current channel status within a predetermined codebook(i.e., a set of precoding matrices) according to a predetermined rankvalue supported by DL transmission. Alternatively, the present inventiondoes not limit the scope of UE assumption in which the UE always selectsa first preferred precoding matrix within the codebook, and a linkquality can be estimated on the assumption that a second or thirdpreferred precoding matrix is selected. As a result, the presentinvention can provide the degree of freedom to precoding matrixselection of the BS. If the UE feeds back precoding information to theBS, the BS may determine the precoding operation to be actually appliedin consideration of feedback information received from the UE althoughthe BS is not limited to the feedback information from the UE. If thenumber of precoding matrices capable of being assumed for link qualityestimation of the UE increases, the range of selectable candidates isincreased even when the BS performs actual precoding so that the degreeof freedom increases in precoding application.

Meanwhile, the UE for use in the legacy wireless communication systemmay calculate/decide CSI on the assumption of DL data (e.g., PDSCH)transmission, and report the calculated/decided CSI. However, CSIreporting on the assumption of transmission of a new control channel(e.g., E-PDCCH) is not defined. In this case, if the UE is configured toreport a PMI to the BS as a CSI feedback of PDSCH, the UE can estimate alink quality on the assumption that the precoding matrix correspondingto the reported PMI is used for E-PDCCH transmission.

In addition, Transmit Rank of E-PDCCH may be limited. For example,although the wireless communication system supports up to Rank #8 for DLdata transmission, only a lower rank (e.g., Rank 1 or 2) may besupported for E-PDCCH transmission. Therefore, when the UE selects a PMIon the assumption of link quality calculation through E-PDCCH, the PMImay be selected within the limited rank only. In more detail, when theUE estimates the E-PDCCH decoding error probability using a PMI (i.e.,PMI on the assumption of PDSCH transmission) reported to the BS, it isassumed that PMI of ‘Rank=2 or higher’ is reported for PDSCH andtransmission rank of the E-PDCCH is limited to ‘1’. In this case,assuming that each of multiple column vectors of the precoding matrixcorresponding to the reported PMI is used for E-PDCCH at the sameprobability, the UE can estimate the E-PDCCH decoding error probability.

On the contrary, the UE may obtain precoding information to be used forRLM of the precoding-based control channel using CRS instead of CSI-RS.For example, if the BS does not configure CSI-RS or if the number ofantenna ports configured in CSI-RS is equal to or less than apredetermined number, it may be difficult to assume precodinginformation applied to the precoding-based control channel using CSI-RS.In this case, assuming that the UE selects a precoding matrixappropriate for a channel state through CRS and the BS uses thecorresponding precoding matrix, the decoding error probability (i.e.,link quality) of E-PDCCH can be estimated.

In accordance with the CRS scheme, the same frequency band is used forUL and DL as in the TDD system, such that the CRS scheme may beefficiently used in the case in which there is a high similarity betweenUL and DL channels. If the UL channel state is similar to the DL channelstate, the BS estimates the UL channel from a sounding reference signal(SRS) transmitted to UL and also estimates the DL channel from ULchannel estimation. The UE decides precoding appropriate for a DLchannel state derived through SRS by the BS, and assumes that thedecided precoding will be applied to DL transmission. Here, the BS canperform DL channel estimation without additional CSI-RS configuration,such that the operation for estimating the decoding error probability ofthe precoding-based control channel using CRS may be helpful to overheadreduction of a reference signal.

In addition, the UE can expect that precoding applied to UL transmissionwill be applied to DL transmission from the BS. Accordingly, the UE mayassume precoding information to be applied to the precoding-basedcontrol channel in consideration of not only the DL channel stateestimated from CRS but also the UL channel state. For example, the UEsupporting UL MIMO transmission may receive the number of layers andprecoding information related to UL transmission through UL schedulinginformation from the BS. If UL and DL are similar to each other, thereis a high probability that UL precoding information will be applied toDL transmission. Accordingly, when the UE assumes precoding informationto be applied to a precoding-based control channel, the UEsimultaneously considers a DL channel state estimated through CRS andprecoding information of the UL channel, such that the UE can assumeprecoding information more approximate to actual application precodinginformation. Of course, if there is a high likelihood between UL and DLon the assumption of precoding information applied to theprecoding-based control channel through CSI-RS, precoding informationapplied to UL is also considered so that the accuracy of precodingassumption can be improved. Accordingly, RLM based on precodinginformation assumption can be correctly carried out.

Embodiment 1-2

When RLM for the precoding-based control channel is performed, the UEassumes that the precoding matrix applied to the precoding-based controlchannel is randomly selected within the codebook so that a link qualityis estimated on the basis of this assumption.

In this case, the codebook considered by the UE may correspond to atransmit (Tx) rank of the precoding-based control channel. For example,assuming that E-PDCCH is transmitted only to Rank #1, the UE assumesthat one of precoding vectors contained in a Rank-1 codebook predefinedfor PMI reporting is randomly selected (e.g., with a uniform or sameprobability) and the selected precoding vector is applied to E-PDCCHtransmission, such that the E-PDCCH decoding error probability isdecided for RLM execution.

This embodiment 1-2 may be more efficiently used for the case in whichthe precoding-based control channel is optimal for a specific UE. Forexample, if a common search space in which system information istransmitted to multiple UEs is configured in the E-PDCCH region,precoding appropriate for reception of multiple UEs should be applied sothat it is difficult for the optimum precoding to be applied to aspecific UE. In this case, assuming that the precoding matrix applied toE-PDCCH is selected from the codebook, the UE can perform RLM.

When the UE assumes an arbitrary precoding matrix of the codebook so asto perform RLM through the precoding-based control channel, resourcesthrough which the precoding-based control channel is transmitted may beconsidered. For example, the UE can assume that one arbitrary precodingmatrix can be applied to total resources used for E-PDCCH transmission.Alternatively, when one E-PDCCH is transmitted through several resourceblocks (e.g., multiple PRBs), one precoding matrix is applied to eachresource block, and the UE can assume that precoding matrices applied toindividual resource blocks are different from each other. For Txdiversity, when one E-PDCCH is transmitted over several resource blocks,the precoding matrix applied to one RB is not used for the most adjacentRB or another precoding matrix orthogonal to the precoding matrixapplied to one RB may be used for the most adjacent RB. To provide Txdiversity within one RB, the UE can assume that the precoding matrix israndomly applied to each REG (Resource Element Group) or each RE of oneRB constructing one E-PDCCH. The UE estimates the decoding errorprobability of the precoding-based control channel on the assumption ofprecoding applied to the precoding-based control channel, such that theUE can perform RLM. In addition, E-PDCCH (specifically, in case of acommon search space) precoding can be actually applied to the BS in thesame manner as in the above assumption.

Embodiment 1-3

When RLM for the precoding-based control channel is performed, the UEassumes that a specific precoding matrix is applied to theprecoding-based control channel so that a link quality is estimated onthe basis of this assumption.

For example, the UE assumes that the BS transmits E-PDCCH using theprecoding vector [1 1 1 . . . 1]^(T) and estimates the E-PDCCH decodingerror probability, such that the UE can perform RLM. Here, the number ofprecoding vector elements assumed by the UE may be identical to thenumber of CRS or CSI-RS antenna ports configured by the BS. Theabove-mentioned example may be applied to ‘E-PDCCH transmit (Tx)rank=1’, and may be used irrespective of the actual Tx rank of E-PDCCH.Alternatively, one precoding vector is pre-assigned to each Tx ranksupported for E-PDCCH, and RLM may be carried out using the precodingvector.

In another example, it is assumed that precoding applied to theprecoding-based control channel is carried out using the Space-Time Code(STC) scheme denoted by Equation 18, so that RLM is carried out on thebasis of this assumption.

$\begin{matrix}\left\{ \begin{matrix}{{{Signal}{on}{{RE}\left( {2n} \right)}} = {\left\lbrack {{S\left( {2n} \right)}{from}{antenna}a} \right\rbrack{{and}\left\lbrack {{S\left( {{2n} + 1} \right)}{from}{antenna}b} \right\rbrack}}} \\{{{Signal}{on}{{RE}\left( {{2n} + 1} \right)}} = {\left\lbrack {{S\left( {{2n} + 1} \right)}{from}{antenna}a} \right\rbrack{{and}\left\lbrack {{- {S^{*}\left( {2n} \right)}}{from}{antenna}b} \right\rbrack}}}\end{matrix} \right. & \left\lbrack {{Equation}18} \right\rbrack\end{matrix}$

In Equation 18, S(k) denotes a k-th E-PDCCH modulation symbol, and S*(k)denotes a conjugate complex number of the k-th E-PDCCH modulation symbolS(k).

Although the STC precoding of Equation 18 is applied to the case inwhich the BS has two antennas (Antenna #a and Antenna #b), the twoantenna ports may correspond to the first two antenna ports from amongthe CRS or CSI-RS antenna ports configured by the BS. That is, if the UEassumes CRS, Antenna #1 and Antenna #b may correspond to Antenna Port #0and Antenna Port #1, respectively. If the UE assumes CSI-RS, Antenna #1and Antenna #b may correspond to Antenna Port #15 and Antenna Port #16,respectively. In addition, it is obvious to those skilled in the artthat the principles proposed by Equation 18 can be extended to two ormore antennas.

Embodiment 1-4

When RLM for the precoding-based control channel is performed, the UEassumes that a UE-specific RS is directly measured and appropriateprecoding is applied to the UE-specific RS, so that a link quality isestimated on the basis of this assumption.

The above-mentioned examples have described a method for performing RLMthrough the precoding-based control channel on the basis of CSI-RS orCRS. If DL transmission from the BS to the UE is not present, theUE-specific RS is not transmitted but CSI_RS and/or CRS can betransmitted. If a link quality of the precoding-based control channel isderived using CSI-RS or CRS, the BS can provide Tx power information(e.g., E-PDCCH Tx power compared to CRS or CSI-RS power) of theprecoding-based control channel to the UE in advance.

This embodiment proposes a method for performing RLM using UE-specificRS. Generally, since the UE-specific RS is valid only in a frequencyregion in which the corresponding channel is transmitted, the BS mayprovide the UE with information related to transmission of theprecoding-based control channel serving as a target of RLM. For example,an RB set to which E-PDCCH serving as an RLM target is transmitted, anantenna port of a UE-specific RS used for E-PDCCH transmission, ascramble sequence parameter, and/or information of a transmit (Tx) modeapplied to E-PDCCH may be applied to the UE. Specifically, Tx-associatedinformation (e.g., RB set, antenna port, etc.) of the precoding-basedcontrol channel serving as the RLM target may be configuredindependently from other Tx-associated information actually applied tothe precoding-based control channel received by the corresponding UEconfigured to detect control information. (That is, the above twoTx-associated information may be identical to or different from eachother.) Alternatively, the above two Tx-associated information may besome parts of Tx-associated information actually applied to theprecoding-based control channel. The UE performs channel estimationusing the UE-specific RS on the basis of the above information, so thatit can estimate/assume precoding information to be applied to a DLchannel. As a result, the UE estimates the decoding error probablity ofthe precoding-based control channel on the basis of theestimated/assumed precoding result, so that the UE can perform RLM.

Embodiment 2

Embodiment 2 proposes a method for performing more accurate RLM inconsideration of resources to which the precoding-based control channelis mapped.

If there are various precoding-based control channel types (e.g.,E-PDCCH) as shown in FIG. 9 , it is determined that RLM is performedonly for one of the precoding-based control channel types, resulting insimplification of UE implementation. For example, the UE may inform theUE of the E-PDCCH type to be used for RLM through higher layer signalingsuch as RRC signaling. Alternatively, the E-PDCCH type to be used forRLM may be predetermined or fixed without additional signaling. Forexample, specific information indicating that only the DL allocationtransmission type (e.g., E-PDCCH of FIG. 9 ) is used for RLM only may bepredetermined as necessary. Alternatively, other information indicatingthat only a transmission type (e.g., E-PDCCH1 of FIG. 9 ) based on afirst slot having a relatively small number of OFDM symbols used forE-PDCCH transmission is used for RLM may be predetermined as necessary.

In the case of RLM based on the precoding-based control channel,predetermined assumption for a time domain to which the precoding-basedcontrol channel is transmitted may be applied to the above RLM. Forexample, it may be assumed that E-PDCCH is transmitted on apredetermined number of OFDM symbols from the viewpoint of RLM. In moredetail, it is possible to use one assumption that all OFDM symbols of aspecific subframe are used for E-PDCCH transmission, or other assumptionthat the remaining OFDM symbols other than first N OFDM symbols (whereN=1, 2, or 3) of a specific subframe are used for E-PDCCH transmission.The above-mentioned description does not indicate that actual E-PDCCHtransmission is performed on specific OFDM symbols, but indicates UEassumption for RLM. Accordingly, although the number of OFDM symbolsused for actual E-PDCCH transmission is changed to another number (dueto a change of a PDCCH length), the UE RLM operation may be constantlyperformed according to the above assumption without being affected bythe changed number. Here, information regarding the number of E-PDCCH TxOFDM symbols to be assumed from the viewpoint of RLM may be transferredto the UE through higher layer signaling such as RRC signaling.Meanwhile, since a limited number of OFDM symbols can be used forE-PDCCH transmission within a special subframe (See FIG. 1(b)) presentbetween DL/UL subframes in the TDD system, the special subframe may beexcluded from a resource region serving as an RLM target.

When CRS and/or CSI-RS may be transmitted within a target subframe ofthe RLM, the number of REs available for the precoding-based controlchannel may be changed to another number. The decoding error probabilityresult may be changed in response to assumption indicating the number ofprecoding-based control channel Tx REs. Accordingly, assumptionregarding the number of REs through which the precoding-based controlchannel can be transmitted is of importance to RLM. RS configuration orRS overhead (the number of REs occupied by CRS and/or CSI-RS) regardingCRS and/or CSI-RS may be changed per subframe, such that the UEoperation can be simplified on predetermined assumption of RSconfiguration or RS overhead for RLM. For example, the UE(optimistically) assumes that CRS and/or CSI-RS are not present in theE-PDCCH transmission region, such that the UE can perform RLM.Alternatively, in order to confirm a radio link quality even when the UEhas a small number of E-PDCCH Tx REs, CRS and/or CSI-RS occupy many REsin the E-PDCCH transmission region (on pessimistic assumption), suchthat RLM may be carried out according to this assumption. From theviewpoint of pessimistic assumption, predetermined RS overhead isassumed in the same manner as in 4-port CRS or 8-port CSI-RS so that RLMmay be carried out. The above CRS and/or CSI-RS configuration and/oroverhead information may be pre-applied to the UE through higher layersignaling such as RRC signaling.

Embodiment 3

Embodiment 3 proposes a method for selectively or simultaneously usingRLM based on the legacy PDCCH and another RLM based on theprecoding-based control channel.

Although the precoding-based control channel (e.g., E-PDCCH) is used asshown in FIG. 9 , the UE may decode the legacy PDCCH. For example, PDCCHincluding scheduling information received by all UEs of the cell can betransmitted on a common search space in the same manner as in systeminformation. Although the UE is regarded as a UE capable of decodingE-PDCCH, some control information can be received through the legacyPDCCH. If the UE decodes both PDCCH and E-PDCCH as described above, itis necessary for a link quality measurement reference to indicate eitherone of PDCCH or E-PDCCH or both of PDCCH and E-PDCCH, so that RLM can becorrectly carried out.

Accordingly, if the UE decodes both PDCCH and E-PDCCH (or if the UE hassuch capability for decoding PDCCH and E-PDCCH), this embodimentproposes a method for allowing the BS and the UE to share a link qualityestimation reference.

For example, the BS may inform the UE of specific information indicatingwhether RLM is performed using one of PDCCH and E-PDCCH through higherlayer signaling such as RRC signaling.

In another example, the UE may perform RLM using both PDCCH and E-PDCCH.For example, if each of one link quality (Q_(PDCCH)) estimated usingPDCCH and another link quality (Q_(E-PDCCH)) estimated using E-PDCCH islower than a predetermined threshold value (Q_(out)), this means that aradio link quality from the viewpoint of RLM is less than the thresholdvalue (Q_(out)) or the occurrence of “out-of-sync” may be determined. Ifat least one of Q_(PDCCH) and QE-PDCCH is less than Q_(out), theoccurrence of “out-of-sync” from the viewpoint of RLM may be decided.Alternatively, a link quality for representing Q_(PDCCH) and Q_(E-PDCCH)is estimated (or a link quality of a mean value is estimated). If thecorresponding representative link quality is less than a predeterminedthreshold value (Q_(out)), the occurrence of “out-of-sync” from theviewpoint of RLM may be determined.

In addition, a predetermined threshold value applied to RLM based onPDCCH may be defined as Q_(out) _(_) ₁, and a predetermined thresholdvalue applied to RLM based on E-PDCCH may be defined as Q_(out) _(_) ₂.In this case, Q_(PDCCH) is compared with Q_(out) _(_) ₁, and Q_(E-PDCCH)is compared to Q_(out) _(_) ₂. If Q_(PDCCH) is less than Q_(out) _(_) ₁and Q_(E-PDCCH) is less than Q_(out) _(_) ₂, the link quality from theviewpoint of RLM may also be determined to be “out-of-sync” from theviewpoint of RLM. Alternatively, Q_(PDCCH) is compared with Q_(out) _(_)₁, and Q_(E-PDCCH) is compared with Q_(out) _(_) ₂. If at least one ofthe two comparison results is less than a threshold value (Q_(out) _(_)₁ or Q_(out) _(_) ₂), a radio link quality from the viewpoint of RLM maybe determined to be “out-of-sync” from the viewpoint of RLM.

Similarly, if both Q_(PDCCH) and Q_(E-PDCCH) are higher than apredetermined threshold value (Q_(in)), this means that a radio linkquality from the viewpoint of RLM is higher than Q_(in) or theoccurrence of “in-sync” may be determined. If at least one of Q_(PDCCH)and Q_(E-PDCCH) is less than Q_(in), the occurrence of “in-sync” fromthe viewpoint of RLM may be decided. Alternatively, a link quality forrepresenting Q_(PDCCH) and QE-PDCCH is estimated (or a link quality of amean value is estimated). If the corresponding representative linkquality is higher than a predetermined threshold value (Q_(in)), theoccurrence of “in-sync” from the viewpoint of RLM may be determined.

In addition, a predetermined threshold value applied to RLM based onPDCCH may be denoted by Q_(in) _(_) ₁, and a predetermined thresholdvalue applied to RLM based on E-PDCCH may be denoted by Q_(in) _(_) ₂.In this case, Q_(PDCCH) is compared with Q_(in) _(_) ₁, and Q_(E-PDCCH)is compared to Q_(in) _(_) ₂. If Q_(PDCCH) is higher than Q_(in) _(_) ₁and Q_(E-PDCCH) is less than Q_(in) _(_) ₂, the link quality from theviewpoint of RLM may also be determined to be “in-sync” from theviewpoint of RLM. Alternatively, Q_(PDCCH) is compared with Q_(in) _(_)₁, and Q_(E-PDCCH) is compared with Q_(in) _(_) ₂. If at least one ofthe two comparison results is less than a threshold value (Q_(in) _(_) ₁or Q_(in) _(_) ₂), a radio link quality from the viewpoint of RLM may bedetermined to be “in-sync” from the viewpoint of RLM.

If a link quality of only one control channel from among PDCCH andE-PDCCH is less than a threshold value (or if the occurrence of“out-of-sync” from the viewpoint of RLM is decided), the UE may informthe BS of this fact through the other control channel (i.e., through ULtransmission controlled by the other channel). The BS having receivedthe above-mentioned information may switch current configuration toanother in such a manner that various information transmitted through acontrol channel having a poor link quality is transmitted throughanother control channel. For example, when the UE performs RLM of the BSwhich transmits control information transmitted within a common searchspace through a PDCCH, if a link quality of PDCCH is less than athreshold value, the UE may inform the BS of the poor link qualitythrough a PUSCH corresponding to a UL grant received through E-PDCCH.The BS having received the above information may perform configurationswitching in such a manner that control information transmitted within acommon search space can be transmitted through E-PDCCH.

Further, the UE may periodically or aperiodically report channel statusinformation (CSI) of a specific control channel (e.g., E-PDCCH) to theUE. Channel status information (CSI) of a specific control channel mayreport either the E-PDCCH decoding error probability at a currentchannel status or the aggregation level satisfying a predetermined errorprobability (e.g., error probability of 1%). For such reporting, the UEmust estimate the E-PDCCH error probability according to a given channelstatus. For this purpose, the above assumption (e.g., PDSCH precodingcan also be applied to E-PDCCH) described in the above-mentionedembodiments may be used. In association with the UE reporting, precodinginformation for channel status information (CSI) of E-PDCCH may beimplemented by reusing other precoding information (or codebook) definedfor PDSCH.

In addition, link quality information (or bits) of E-PDCCH may becontained in CSI reporting of PDSCH.

Embodiment 4

Embodiment 4 proposes a method for correctly and efficiently performingRLM when the CoMP scheme is applied to a control channel. If the BSperforms the CoMP operation, inter-cell interference (ICI) of a controlchannel (e.g., PDCCH and/or E-PDCCH) can be reduced. JT, CS/CB, DCS,etc. may be used as the CoMP scheme applicable to the BS.

If the UE operation is configured according to the CoMP operation of theBS, the CoMP operation of the BS is assumed and a link quality isestimated while the UE performs RLM. For example, if the UE isconfigured to perform JT feedback (e.g., if inter-cell CSI for enablingsignals transmitted from multiple cells to be sufficiently combined witheach other is fed back, the UE assumes that a control channel (PDCCHand/or E-PDCCH) is transferred from cell(s) participating in JT so as toestimate a link quality.

For the CoMP operation, the BS estimates interference of a specificresource (e.g., a resource corresponding to zero-power CSI-RSconfiguration, as a restricted measurement resource), such that the BSmay enable the UE to perform CQI calculation. In this case, the specificresource may correspond to a resource element (RE) capable of minimizingor muting Tx power of the corresponding cell, and the UE can measureinterference from other cells using the corresponding resource. Ifspecific resources for interference estimation are configured asdescribed above, the UE may derive the link quality for RLM usinginterference estimated for the specific resource.

In addition, if multiple interference resources (i.e., resources to beused for interference measurement) are configured to calculate multipleCQIs, the BS may inform the UE of specific information indicating whichone of multiple interference resources will be used for RLM execution.Specific information indicating interference resources related to suchRLM execution may be provided to the UE through higher layer signalingsuch as RRC signaling. Alternatively, priority of multiple interferenceresources is predetermined, and interference resources having thehighest priority (e.g., the lowest index) may be related to RLM.

The above-mentioned UE RLM operation may be restrictively applied to acontrol channel structure to which the CoMP control channel transmissionand the interference measurement resource configuration are applied. Forexample, the above-mentioned UE RLM operation may be restrictivelyapplied to E-PDCCH having a format similar to that of PDSCH.

Meanwhile, if DCS is applied to a control channel (PDCCH and/orE-PDCCH), the UE performs blind decoding (BL) of a control channel frommultiple cells participating in DCS so that the UE can receive controlinformation. In this case, the UE estimates a link quality of each cell,and calculates the final link quality using the estimated result, suchthat the UE can perform RLM on the basis of the final link quality.Embodiment 4 shows a method for allowing the BS and the UE to share apredetermined rule related to link quality estimation when the UEdecodes a PDCCH from multiple cells.

For example, the UE may inform the UE of specific information indicatingwhich cell will be used for RLM through higher layer signaling such asRRC signaling, and the UE performs RLM for the indicated cell(s) so thatit may inform the BS of the RLM result. Alternatively, after the UEperforms RLM or each cell, the UE may report the RLM result to a primaryserving cell. The UE may report the RLM results for individual cells inparallel, or may simultaneously report the RLM results. In accordancewith the scheme for integrating measurement results of a plurality ofcells, if all link qualities of multiple cells are less than Q_(out),this means that a radio link quality from the viewpoint of RLM becomesdeteriorated or the occurrence of “out-of-sync” may be decided.Alternatively, if at least one of link qualities of multiple cells isless than Q_(out), the occurrence of “out-of-sync” from the viewpoint ofRLM may be decided. Similarly, if all link qualities of multiple cellsare higher than Q_(in), this means that a radio link quality from theviewpoint of RLM becomes improved or the occurrence of “in-sync” may bedecided. Alternatively, if at least one of link qualities of multiplecells is higher than Q_(in), the occurrence of “in-sync” from theviewpoint of RLM may be decided.

If a link quality of only one cell from among multiple cells is lessthan a threshold value (or if “out-of-sync” from the viewpoint of RLM isdecided), the UE may inform the BS of this fact through the other cell(i.e., through UL transmission controlled by the other cell). The BShaving received this reporting information may switch currentconfiguration to another in such a manner that various informationtransmitted through a control channel having a poor link quality istransmitted through a control channel of another cell. For example, whenthe UE performs RLM of the BS which transmits control informationtransmitted within a common search space on Cell #1, if a link qualityof a control channel of Cell #1 is less than a threshold value, the UEmay inform the BS of this fact through a PUSCH corresponding to a ULgrant received on Cell #2. The BS having received the above informationmay perform configuration switching in such a manner that controlinformation transmitted within a common search space can be transmittedthrough a control channel of Cell #2.

Alternatively, the operation for reporting the RLM results of individualcells in parallel or collectively may cause unexpected load to the UE,such that a method for simplifying this reporting operation may beconsidered. For example, the BS selects only one representative cellfrom among multiple cells through higher layer signaling such as RRCsignaling, and may inform the UE of a specific command for performingRLM of only one representative cell (e.g., 1210 of FIG. 12).Specifically, if each cell transmits CSI-RS and the UE performs CSImeasurement and RLM on the basis of the CSI-RS transmission result, theBS selects a representative CSI-RS configuration through higher layersignaling such as RRC signaling (e.g., 1205 of FIG. 12), and commandsthe UE to perform RLM (e.g., 1210 of FIG. 12) on the basis of theestimated link quality using CSI-RS caused by the corresponding CSI-RSconfiguration. The above-mentioned scheme can be more efficiently usedwhen multiple transmission points (TPs) share the same cell ID andindividual TPs are identified from each other according to differentCSI-RS configurations (for example, CSI-RS pattern, Tx period, offset,antenna port, etc.). For example, in association with multiple TPsconfigured to share the same cell ID, one optimum TP appropriately for acontrol channel (e.g., PDCCH and/or E-PDCCH) which is transmitted to theUE according to Tx power of each TP and the distance from each TP to theUE may be determined. In this case, if CSI-RS of the determined one TPis set to a representative CSI-RS, the UE may perform RLM for only therepresentative CSI-RS without the necessity of performing RLM inparallel to multiple CSI-RSs.

In accordance with the above-mentioned example described on the basis ofmultiple cells, multiple CSI-RS configurations may be given to thecorresponding UE from the viewpoint of one UE. For example, the BS mayinform (e.g., 1205 of FIG. 12) the UE of specific information indicatingwhich CSI-RS configuration(s) from among multiple CSI-RS configurationswill be used for RLM (e.g., 1210 of FIG. 12) execution. The UE mayreport the RLM result based on multiple CSI-RS configurations inparallel or collectively. In the case in which at least one or all oflink qualities estimated using multiple CSI-RS configurations is lessthan Q_(out), this case may be denoted by “out-of-sync” (e.g.,out-of-sync at 1215 of FIG. 12). In the other case in which at least oneor all of link qualities estimated using multiple CSI-RS configurationsis higher than Q_(in), this case may be denoted by “in-sync” (e.g.,in-sync at 1215 of FIG. 12). Alternatively, under the condition that arepresentative CSI-RS related to RLM from among multiple CSI-RSconfigurations may be decided, RLM may be carried out using thecorresponding representative CSI-RS only.

Embodiment 5

Embodiment 5 shows examples to which UE RLM operations shown in theabove-mentioned embodiments are applied.

Assuming that the precoding-based control channel (e.g., E-PDCCH)operates on the basis of UE-specific RS, the UE may measure/estimateSINR of the resource region to which the UE-specific RS is transmitted.Meanwhile, a reference threshold value (e.g., Q_(out)) of RLF may beindependently configured according to the Tx scheme (e.g., one Txdiversity or one beamforming) applied to E-PDCCH. The RLF thresholdvalue according to the Tx scheme may be constructed in the form of alookup table, and this lookup table may include SINR informationcorresponding to Q_(out) defined for each Tx scheme. In addition, theabove-mentioned lookup table may further include specific information inwhich elements affecting the E-PDCCH coding rate are considered, andthis information may be represented by an effective coding rate. Forexample, the elements affecting the E-PDCCH coding rate may includetransmission or non-transmission of CRS and CSI-RS, the number ofantenna ports of CRS and CSI-RS, the number of OFDM symbols for PDCCHtransmission, and transmission or non-transmission of other principalchannel/signals (BCH, PSS/SSS, paging, etc.). In addition, if theE-PDCCH transmission condition (for example, a transmission scheme,etc.) of the UE and the SINR measurement/estimation values of theUE-specific RS are determined according to the schemes described in theabove-mentioned embodiments, the UE may perform RLM measurement (inwhich the E-PDCCH link quality is higher/less than Q_(out)) of a currentsubframe. The following Table 3 shows an example of the lookup tablerelated to Q_(out).

TABLE 3 Effective Transmit diversity mode Beamforming mode Coding RateECR₀₋₀ ECR₀₋₁ ECR₀₋₂ ECR₁₋₀ ECR₁₋₁ ECR₁₋₂ Q_(out) (SINR) SINR₀₋₀ SINR₀₋₁SINR₀₋₂ SINR₁₋₀ SINR₁₋₁ SINR₁₋₂

In Table 3, ECR₀₋₀ is a coding rate to which the remaining signals otherthan E-PDCCH and UE-specific RS are not applied. ECR₀₋₁ is a coding ratefor use in the case in which 4-port CRS is additionally transmitted, andECR₀₋₂ may correspond to a coding rate for use in the other case inwhich SCH is additionally transmitted. If the set of RSs related toE-PDCCH, the number of symbols used in the PDCCH region, a condition ofthe E-PDCCH type, etc. are fixed in advance, the E-PDCCH effectivecoding rate for various conditions need not be configured, so that thelookup table can be reduced in size.

When the link quality measured/estimated on the basis of the controlchannel (PDCCH and/or E-PDCCH) of RLM is compared with Q_(in), thelookup table in which the control channel transmission condition similarto that of Table 3 is considered is constructed, and RLM measurement(i.e., comparison between Q_(in) and a control channel link quality) maybe performed in consideration of the lookup table in which the effectivecoding rate is considered. Accordingly, the RLM operations can be moreefficiently and correctly carried out in various conditions.

FIG. 10 is a flowchart illustrating an RLM method according to oneembodiment of the present invention.

Referring to FIG. 10 , the UE may receive the precoded control channel(e.g., E-PDCCH). The precoding information applied to the precodedcontrol channel may be transparent to the UE. The UE may demodulate theprecoded control channel on the basis of the channel estimated from theUE-specific RS.

In step S1020, the UE may assume the precoding applied to the precodedcontrol channel, and may measure a radio link quality of the precodedcontrol channel on the basis f this assumption.

The above-mentioned RLM method shown in FIG. 10 may be implemented suchthat above described various embodiments of the present invention may beindependently applied or two or more embodiments thereof may besimultaneously applied and a repeated description is omitted forclarity.

FIG. 11 illustrates a configuration of a transceiver according to anembodiment of the present invention.

Referring to FIG. 11 , a transceiver 1110 according to an embodiment ofthe present invention may include a reception (Rx) module 1111, atransmission (Tx) module 1112, a processor 1113, a memory 1114 and aplurality of antennas 1115. The reception (Rx) module 1111 may beconfigured to receive various signals, data and information from anexternal device. The transmission module 1112 may be configured totransmit various signals, data and information to the external device.The processor 1113 may control overall operation of the transceiver 1110and may be configured to execute a function of processing informationtransmitted/received between the transceiver 1110 and the externaldevice. The memory 1114 may store the processed information for apredetermined time and may be replaced by a component such as a buffer(not shown). The antennas 1115 can support MIMO transmission (Tx) andreception (Rx).

The transceiver 1110 according to an embodiment of the present inventionmay be configured to perform Radio Link Monitoring (RLM). The processor1113 of the transceiver 1110 may be configured to receive the precodedcontrol channel through the Tx module. In addition, the processor 1113may be configured to estimate a radio link quality of the receivedprecoded control channel. In this case, the above-mentioned radio linkquality may be estimated on the basis of UE assumption of the procodingoperation applied to the precoded control channel.

The transceiver 1110 may be implemented such that the above-describedembodiments of the invention can be independently applied thereto or twoor more of the embodiments can be simultaneously applied thereto anddescriptions of redundant parts are omitted for clarity.

The transceiver 1110 shown in FIG. 11 may be a UE configured to performRLM of a downlink from a BS or a relay, or may be a relay configured toperform RLM of a downlink from a BS.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof.

In a hardware configuration, the methods according to the embodiments ofthe present invention may be achieved by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. For example, software code may be stored in a memory unitand executed by a processor. The memory unit is located at the interioror exterior of the processor and may transmit and receive data to andfrom the processor via various known means.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein. Also, it will be obvious to thoseskilled in the art that claims that are not explicitly cited in theappended claims may be presented in combination as an exemplaryembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

INDUSTRIAL APPLICABILITY

The above-mentioned embodiments can be applied to various mobilecommunication systems.

The invention claimed is:
 1. A method for performing radio linkmonitoring (RLM) by a user equipment (UE) in a wireless communicationsystem, the method comprising: receiving a precoded control channel froma base station (BS); and estimating a radio link quality of the receivedprecoded control channel, wherein the radio link quality is estimated onthe basis of UE assumption associated with precoding applied to theprecoded control channel, wherein the UE assumption indicates that theprecoding is carried out using a space-time code scheme, wherein a2n^(th) resource element of the precoded control channel includes a2n^(th) modulation symbol transmitted from a first antenna of the BS anda (2n+1)^(th) modulation symbol transmitted from a second antenna of theBS, wherein a (2n+1)^(th) resource element of the precoded controlchannel includes a (2n+1)^(th) modulation symbol transmitted from thefirst antenna and a negative conjugate of a 2n^(th) modulation symboltransmitted from the second antenna, and wherein n is an integer equalto or greater than
 0. 2. The method according to claim 1, whereininformation regarding the precoding applied to the precoded controlchannel is transparent to the user equipment (UE).
 3. The methodaccording to claim 1, wherein the precoded control channel is anEnhanced-Physical Downlink Control Channel (E-PDCCH).
 4. A userequipment (UE) for performing radio link monitoring (RLM) in a wirelesscommunication system, comprising: a reception (Rx) module configured toreceive a downlink signal from a base station (BS); a transmission (Tx)module configured to transmit an uplink signal to the BS; and aprocessor configured to control the UE including the reception (Rx)module and the transmission (Tx) module, wherein the processor receivesa precoded control channel through the reception (Rx) module, estimatesa radio link quality of the received precoded control channel, whereinthe radio link quality is estimated on the basis of UE assumptionassociated with precoding applied to the precoded control channel,wherein the UE assumption indicates that the precoding is carried outusing a space-time code scheme, wherein a 2n^(th) resource element ofthe precoded control channel includes a 2n^(th) modulation symboltransmitted from a first antenna of the BS and a (2n+1)^(th) modulationsymbol transmitted from a second antenna of the BS, wherein a(2n+1)^(th) resource element of the precoded control channel includes a(2n+1)^(th) modulation symbol transmitted from the first antenna and anegative conjugate of a 2n^(th) modulation symbol transmitted from thesecond antenna, and wherein n is an integer equal to or greater than 0.5. A method of performing a radio link monitoring by a user equipment(UE) configured with a plurality of cells in a wireless communicationsystem, the method comprising: receiving, from a base station (BS),information regarding a set of resources of a first cell for the radiolink monitoring; performing the radio link monitoring for the first cellonly excluding other cells among the plurality of cells, based on theset of resources of the first cell; and determining whether a result ofthe radio link monitoring is ‘out-of-synchronization’ or‘in-synchronization’ based on a first threshold value (Q_(out)) and asecond threshold value (Q_(in)), wherein the result of the radio linkmonitoring is determined to be ‘out-of-synchronization’ based on thatall radio link qualities of all resources in the set of resources of thefirst cell other than the other cells is less than the first thresholdvalue (Q_(out)), wherein the result of the radio link monitoring isdetermined to be ‘in-synchronization’, based on that any radio linkquality of any resource in the set of resources of the first cell otherthan the other cells is greater than the second threshold value(Q_(in)), and wherein the information regarding the set of resourcesincludes information for configuring that at least one signal includinga first channel state information-reference signal (CSI-RS) is set as areference signal (RS) for the radio link monitoring from among aplurality of CSI-RSs configured in the UE.
 6. The method of claim 5,wherein the radio link monitoring performed based on the first CSI-RS isrelated to an error rate of a control channel which is determined basedon a predetermined number of control channel symbols and a controlchannel-to-CSI-RS energy ratio.
 7. A non-transitory computer-readablemedium storing program codes for performing the method of claim
 5. 8. Auser equipment (UE) configured with a plurality of cells in a wirelesscommunication system, the UE comprising: a transceiver configured totransmit and receive a radio signal with a base station (BS); and aprocessor configured to control the transceiver, wherein the processoris further configured to: receive, from a base station (BS), informationregarding a set of resources of a first cell for the radio linkmonitoring; perform the radio link monitoring for the first cell onlyexcluding other cells among the plurality of cells, based on the set ofresources of the first cell; and determine whether a result of the radiolink monitoring is ‘out-of-synchronization’ or ‘in-synchronization’based on a first threshold value (Q_(out)) and a second threshold value(Q_(in)), wherein the result of the radio link monitoring is determinedto be ‘in-synchronization’, based on that any radio link quality of anyresource in the set of resources of the first cell other than the othercells is greater than the second threshold value (Q_(in)), and whereinthe information regarding the set of resources includes information forconfiguring that at least one signal including a first channel stateinformation-reference signal (CSI-RS) is set as a reference signal (RS)for the radio link monitoring from among a plurality of CSI-RSsconfigured in the UE.
 9. The UE of claim 8, wherein the radio linkmonitoring performed based on the first CSI-RS is related to an errorrate of a control channel which is determined based on a predeterminednumber of control channel symbols and a control channel-to-CSI-RS energyratio.